In some fields of application, frequency synthesizers are not known for quality of their spectrum, especially as regards the spectral noise components situated more than a few kHz from the carrier.
Because of this, they are barred from a certain number of applications, such as measurement of the signal/noise ratio on receivers, the control of UHF transmitters and the use of the synthesizer as a local oscillator for a spectrum analyser with a high dynamic characteristic.
It would be desirable, for such applications, to be able to make a synthesizer covering a range of high frequencies going up to several hundrer MHz with a noise level, at 12.5 kHz from the carrier, equal to -140 db/Hz at the most.
Now the synthesis of high frequencies is generally effected by comparison of the frequency of an harmonic component having a standard frequency of the order of 10 MHz with the output frequency of a voltage-controlled oscillator (VCO), and by tuning the latter. While it is known how to make a crystal master oscillator which generates the standard frequency with the desired spectral purity, multiplication of this standard frequency by factors of several tens multiplies the noise in the same ratio and makes it quite incompatible with the application envisaged.